The present invention concerns a heat recovery system of a boiler with a CO2 capture system.
In recent years, reduction of CO2 emission has been demanded world wide for suppressing global warming. Particularly, exhaust gases discharged from equipment such as coal-fired boilers, gas turbines, and chemical plants contain CO2 as much as several % or more and a method of separating and capturing CO2 has been demanded.
An exhaust gas processing system of a coal-fired boiler includes an NOx reduction system provided downstream of a boiler for reduction and detoxification of nitrogen oxides (hereinafter referred to as NOx), an air heater provided downstream thereof for cooling an exhaust gas using air as a coolant, a heat exchanger provided downstream thereof for cooling the exhaust gas using water as a coolant, a dust removal system provided downstream thereof for removing dusts and soots in the exhaust gas, a desulfurization system provided downstream thereof for absorption and detoxification of sulfur oxide (hereinafter referred to as SOx), and a heat exchanger provided downstream thereof for warming the exhaust gas (for example, refer to IHI Technical Report Vol. 45, No. 1 (2005-3) (hereinafter referred to as Non-Patent Document 1). Water heated by the heat exchanger downstream of the air heater is utilized as a heat source for the heat exchanger provided downstream of the desulfurization system. Hereinafter, the former heat exchanger is referred to as a heat recovery heat exchanger and the latter heat exchanger is referred to as a re-heating heat exchanger. The re-heating heat exchanger is provided for preventing steams discharged out of a chimney from forming white smoke that results in visual pollution. In a place where such regulation is imposed, installation of the re-heating heat exchanger is legally obliged.
Further, CO2 in the exhaust gas can be separated and captured by providing a CO2 capture system downstream of a desulfurization system. As a method of separating and capturing CO2 in the exhaust gas, a method of absorbing CO2 in a CO2 adsorbing column by using an amine solution including MEA (monoethanol amine), etc. which is applied to separation and capture of CO2 in the exhaust gas from a boiler or a gas turbine. For improving a CO2 capturing efficiency, various amine compounds have been proposed (for example, refer to Japanese Patent Publications Nos. 3761960 and 3771708). The amine compound has a high ability of separating and capturing CO2. However, since the amine compound is poisoned by oxygen or SOx, etc. in the exhaust gas or scatters partially from the CO2 absorption column, supplementation of the amine compound is necessary to increase the cost.
Then, a CO2 capture system using a CO2 solid sorbent which is less poisoned by oxygen, SOx, etc. in the exhaust gas and less scatters has been studied. For example, a system of having four columns packed with a CO2 solid sorbent and capturing CO2 by four steps of (1) sorbing CO2 by a sorbent, (2) purging the inside of the column, (3) desorbing CO2 from the sorbent, and (4) cooling the sorbent is disclosed in NEDO Report, Hei 14 (2002), Development for Effective Utilization of Technique of Fixing Carbon Dioxide for Practical Use, and Development for CO2 Separating and Capturing Technique by Chemical Adsorption, by Shikoku Research Institute (2003-3) (hereinafter referred to as Non-Patent Document 2).
Further, a technique capable of downsizing the system by constructing a rotational driving type CO2 capture system using a CO2 solid sorbent is described in “CO2 Separation/capture and Store/Isolation Technique”, published from NTS (2009) 76 (hereinafter referred to as Non-Patent Document 3).
On the other hand, for the heat efficiency of a boiler, a heat efficiency of an electric power generation system is improved generally by re-heating a condensate generated from a condenser provided downstream of a steam turbine by steams extracted from the steam turbine.
In a case where the re-heating exchanger is not present, heat recovered by the heat recovery heat exchanger can be utilized in other uses. For example, Japanese Unexamined Patent Application Publication No. S60 (1985)-227845 describes a method of heating a condensate by using heat recovered from a boiler exhaust gas by a heat recovery heat exchanger thereby improving the heat efficiency of the boiler (refer to FIG. 5).
Further, Japanese Patent Unexamined Application Publications Nos. H03 (1991)-193116 and 2010-240617 describe that the heat efficiency of a boiler is improved by heating a condensate utilizing the heat of captured CO2 which is generated from a CO2 capture system using an amine solution.